Modern oil well drilling technology has allowed operators to drill complex extended reach wells, horizontal wells, and multilateral wells that have lateral branches from a mother wellbore. These innovations have allowed operators to increase production from a single well many fold over traditional vertical oil wells. The so-called “MRC—Maximum Reservoir Contact” wells and “ERC—Extreme Reservoir Contact” wells” comprise a mother wellbore from which a large number of horizontal lateral wellbores are drilled. The mother wellbore and horizontal laterals penetrate the oil bearing layers and are able to drain a large areal extent of the oil reservoir. The lateral wellbores may be thousands of feet in length.
The many lateral wellbores from one mother wellbore may exploit a single oil zone, in which case they are within the same formation attached to the mother wellbore at essentially one depth. However, it is also possible to drill the laterals in two or more oil zones at different depths in the earth. In either case, the flows from the different laterals are comingled in the mother wellbore.
These types of wells not only significantly increase the rate of oil production, but can also increase the total recovery factor by reducing the pressure drop between the formation and the wellbores. By reducing the pressure drop, water underlying the oil zone is less likely to break through the oil layer and enter a wellbore. Water being generally much less viscous than oil, once water enters the well, it tends to significantly reduce the production of oil. Hence, maintaining low pressure drops over a large extent of the oil reservoir, thus maintaining oil production, can significantly improve the economics of an oil field.
As long as all of the laterals are producing oil, and none are producing much water, the well operation is efficient. However, if water enters one of the laterals, it may flood the mother wellbore and thus greatly reduce the oil flowing from the other laterals into the mother wellbore. Once this happens, the entire well may no longer be economical. Thus, it is desirable to monitor the pressure in the laterals, to monitor the flow of oil and water into each of the laterals, and to have some means of controlling the pressures and some means for reducing the water influx. For example, pressure gauges can be deployed in the mother wellbore and lateral wellbores to monitor pressures. Measuring the resistivity of the fluids in the wellbores can be used to detect water influx. Valves may be deployed in the other wellbore or laterals to choke flow or to shut flow off entirely. If sensors and valves are to be deployed in the lateral wells, then they must have a means for communication to the surface via the mother wellbore, and must have a power source to operate the sensors and valves. Wells that have downhole sensors, valves, and a communication and control system between the reservoir and the surface to monitor and enhance production are known as “intelligent wells”.
Hardware that is deployed in the mother wellbore and/or in the laterals is called the “completion”. The mother wellbore completion may comprise a casing or a liner cemented into the formation, or it may simply be an open borehole. The mother wellbore may also contain tubing which is run inside the casing, liner, or open hole. Packers can be used to isolate the tubing from the casing, so as to force the produced fluids to flow inside the tubing to surface. Packers can also be used in the lateral wells to isolate flow from different sections along the length of the lateral well. Valves in the lateral wells can then be used to reduce or shut-off flow from a section of the lateral that is producing too much water.
Lateral wellbores can be connected to the mother wellbore in a variety of ways with different types of junctions. Multilateral junctions are classified according to levels of increasing performance, complexity and cost, from level 1 (the simplest and least expensive) to level 6 (the most expensive but providing the greatest pressure and mechanical integrity). A level 1 junction is an openhole lateral from an openhole mother wellbore with no mechanical or hydraulic junction. This level is applicable in consolidated formations that do not require casing or liners (a well can be cased with a casing or a liner, a casing extends to the surface, while a liner does not, otherwise they serve the same function). In a level 2 junction, the mother wellbore is cased and cemented, but the lateral wellbore is open. Level 2 junctions are more common than level 1 because they offer greater flexibility and because good technology is available. Level 3 junctions have cased and cemented mother wellbores and lateral wellbores with liners, but the lateral liner is not cemented. In some level 3 multilateral completions, the lateral liner is hung-off the mother wellbore casing. This requires the very accurate placement of the lateral liner with respect to the mother wellbore. In a level 4 junction, both the mother wellbore casing and the lateral liners are cemented. A level 5 junction provides pressure and mechanical integrity using packers and tubing in the both lateral and the mother wellbores. A level 6 multilateral junction is a solid metal junction that is part of the mother wellbore casing. The level 6 junction provides the highest degree of pressure and mechanical integrity.
Providing both power and communications across the different level junctions is an unsolved problem. Some companies provide wireless communications across a junction, but power has to be supplied either by a turbine located in the lateral, or by vibration harvesting (e.g. using piezoelectric crystals) and a rechargeable battery located in the lateral. Alternatively, the completion in the lateral could be provided with long life batteries which are periodically replaced. In each of the above scenarios, however, there are serious drawbacks. A turbine or vibration harvester requires significant flow in the lateral, and may even create a pressure drop that reduces oil production. Because turbines have moving parts, they would have long term reliability and maintenance issues. Rechargeable batteries are notoriously unreliable in a high temperature environment, and would need to be replaced periodically, as would conventional downhole batteries. Well intervention to replace batteries is a very expensive operation, which typically requires production from the entire well to be stopped during the operations. Interrupting production may even result in damaging the formation so that the production rate is permanently reduced.